Unsaturated polyesters are typically used in resin systems for the manufacture of fiber reinforced thermoset plastics. These resin systems generally consist of an unsaturated polyester dissolved in a polymerizable ethylenically unsaturated monomer such as styrene. Typically, the polyester is formed by reacting an unsaturated diacid or anhydride with a nearly equivalent amount of dihydric alcohol at temperatures above about 200.degree. C. for several hours. Maleic anhydride is the most common acid component utilized. The dihydric alcohols which are commonly used include ethylene glycol, 1,2-propylene glycol, dipropylene glycol, diethylene glycol, and the like. Modifying amounts of other diacids, such as phthalic acid, isophthalic acid, terephthalic acid, or adipic acid are also commonly employed. Phthalic anhydride is also often used. Unsaturation which is provided by maleate or fumarate groups within the backbone of the polyester takes part in crosslinking or curing the resin system. Resin systems containing unsaturated polyesters are considered among the least expensive materials suitable for the manufacture of a variety of fiber reinforced products.
Much effort has recently been directed toward the development of alternative polyester compositions with potentially improved economics. To be a viable alternative, such polyester compositions should exhibit the highly desirable properties of conventional unsaturated polyester resin systems. Modification of unsaturated polyesters with olefins such as dicyclopentadiene has been investigated as one method for reducing the cost of unsaturated polyester resin systems.
Investigators have found that incorporation of dicyclopenatadiene results in two structurally different types of polyesters. See, for example, D. L. Nelson, Considerations: Dicyclopentadiene in Polyester Resins, 36th Annual Conference, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Feb. 16-20, 1981, Session 7-E, pages 1-7. One polyester type contains ester and/or ether groups resulting from carboxyl or hydroxyl addition to one of the dicyclopentadiene double bonds. Such ester and/or ether addition groups have the formula ##STR1## The other polyester type contains Diels-Alder reaction groups resulting from dimer cracking and subsequent reaction of cyclopentadiene with a dieneophile such as maleic acid. Such Diels-Alder groups have the formula ##STR2## when maleic acid is the dieneophile. Combinations of the above groups may also be incorporated in polyesters.
U.S. Pat. Nos. 3,883,612, 3,933,757, 3,986,992, 4,035,439 and Reissue No. 29,555, all assigned to SCM Corporation, disclose thickened, low shrink molding compositions containing a dicyclopentadiene-modified polyester polymer obtained by the following steps: (1) reacting a glycol, e.g., propylene glycol, and an unsaturated dibasic acid, e.g., maleic acid, at a temperature of about 150.degree. C. to yield an acid terminated partial copolymer; (2) reacting dicyclopentadiene with the partial copolymer prepared in step (1) at a temperature of about 150.degree. C. to yield a prepolymer; (3) reacting additional glycol with the prepolymer of (2) at a temperature of about 200.degree. C. to yield a dicyclopentadiene polyester polymer; and (4) adding styrene to the dicyclopentadiene polyester polymer. Sheet molding compounds (SMC) and bulk molding compounds (BMC) were prepared from certain of the molding compositions.
U.S. Pat. Nos. 4,233,432 and 4,246,367, both assigned to United States Steel Corporation, disclose dicyclopentadiene modified polyester resins and a method for preparation thereof. The resins are prepared by reacting maleic anhydride and water with dicyclopentadiene at a temperature of about 90.degree. C. to 150.degree. C. to give a maleic half ester of dicyclopentyl alcohol, and thereafter reacting a glycol. e.g., propylene glycol, at a temperature of about 210.degree. C. to form the resultant unsaturated polyester.
U.S. Pat. No. 4,233,413, assigned to Hitachi Chemical Company, Ltd., discloses low shrink resin compositions containing an unsaturated polyester obtained by reacting dicyclopentadiene with an alpha, beta-unsaturated dibasic acid, e.g., maleic acid, or reacting dicyclopentadiene with an alpha, beta-unsaturated dibasic acid anhydride, e.g., maleic anhydride and water at 150.degree. C. or lower to yield a partially esterfied dicyclopentadiene carboxylic acid, which is further reacted with at least one polyhydric alcohol such as a glycol. e.g., propylene glycol, at a temperature of from 150.degree. C.-210.degree. C. This patent discloses cast articles prepared from the resin compositions.
U.S. Pat. No. 4,224,430, assigned to Hitachi Chemical Company, Ltd., discloses high solids resin compositions containing one or more oligomers prepared by reacting dicyclopentadiene with an alpha, beta-unsaturated hydrocarbon, e.g., maleic acid or maleic anhydride, at a temperature of from 100.degree. C.-140.degree. C. to yield a monobasic acid, which is further reacted with a polyhydric alcohol, e.g., diethylene glycol, at a temperature of from 180.degree.-220.degree. C. Cast articles were prepared from the resin compositions.
U.S. Pat. Nos. 4,029,848 and 4,148,765, both assigned to Dow Chemical Company, disclose resin compositions containing an unsaturated polyester obtained by reacting (1) a glycol, e.g., propylene glycol, (2) an olefinically unsaturated dicarboxylic acid or anhydride, e.g., maleic anhydride, (3) a saturated dicarboxylic acid or anhydride, e.g., phthalic anhydride, and (4) dicyclopentadiene at a temperature of about 140.degree. C. for a period of time and thereafter at a temperature of about 200.degree. C. for an additional period of time. Glass laminates were prepared from certain of the resin compositions by a hand roller technique.
In the above prior art methods, dicyclopentadiene is generally initially reacted with an unsaturated diacid or anhydride and/or a dihydric alcohol at temperatures of from about 90.degree. C. to about 150.degree. C. to give partial polyesters containing primarily the dicyclopentadiene addition groups described above. When maleic acid or anhydride is employed in such methods, the resulting polyester contains predominantly the maleate isomer configuration at these low reaction temperatures. The polyesters are generally further reacted for additional periods of time at temperatures of from about 150.degree. C. to about 220.degree. C. with the addition of more unsaturated diacid or anhydride or dihydric alcohol. At these high reaction temperatures, polyesterification occurs. Isomerization of the maleate to fumarate configuration occurs simultaneously with polyesterification. The fumarate isomer configuration is preferred for desirable resin performance in the manufacture of thermoset articles, in particular, the rapid copolymerization of the polyester with styrene.
U.S. Pat. Nos. 3,166,434 and 3,340,327 disclose resin compositions containing an unsaturated polyester obtained by reacting (1) an unsaturated dicarboxylic acid containing a major molar proportion of fumaric acid, (2) a glycol containing a major molar proportion of polyoxyalkylene glycol and (3) dicyclopentadiene at a temperature up to about 215.degree. C. in the absence of a catalyst (see Example 1 in both patents). Diels-Alder reaction products accompany this polyesterification reaction. Coatings were prepared from certain of the resin compositions.
At the high reaction temperatures utilized in the prior art processes, both reacted and unreacted dicyclopentadiene become increasingly susceptible to fragmentary side reactions. For example, the remaining double bond in reacted dicyclopentadiene becomes increasingly susceptible to esterification and etherification reactions. Also, any unreacted dicyclopentadiene can readily undergo dimer cracking at temperatures above about 150.degree. C. and thereby provide for the formation of Diels-Alder groups which can be incorporated into the structure of the polyester.
As a result of high reaction temperatures, polyesters prepared according to the above prior art methods have a comparatively large polydispersity as determined by conventional analytical methods such as gel permeation chromatography. It would be highly desirable to prepare dicyclopentadiene modified polyesters by a process in which dicyclopentadiene could be selectively reacted with a polyester having fumarate isomer configuration at low reaction temperatures, and thereby effectively preserve the dicyclopentadiene from fragmentary side reactions as potentially occur at high reaction temperatures. The selective reactivity of dicyclopentadiene with a polyester having fumarate isomer configuration at low reaction temperatures can provide polyesters having comparatively narrow molecular weight distributions. Narrow molecular weight distributions are highly desirable for applications requiring low viscosity resin systems such as those used in injection molding and in high solids coatings.
It has been found as a result of this invention that dicyclopentadiene can be selectively reacted with a polyester having fumarate half ester configuration at low reaction temperatures by utilizing a non-oxidizing acid catalyst having a non-nucleophilic anion. Dicyclopentadiene need no longer be exposed to high reaction temperatures utilized in the prior art methods. The dicyclopentadiene modified polyesters of this invention contain predominantly terminal ester groups resulting from selective carboxyl addition to one of the dicyclopentadiene double bonds. The formation of Diels-Alder reaction products is selectively minimized by utilizing dicyclopentadiene at low reaction temperatures. The dicyclopentadiene modified polyesters have a well defined structure and narrow molecular weight distribution.
There appears to be no prior art directed to the addition of dicyclopentadiene at low reaction temperatures to polyesters containing predominantly fumarate half ester terminal groups.